Some extensions fail to install with an error "This add-on could not be installed because it appears to be corrupt" although "Load Temporary Add-on" at about:debugging works fine for them. As an example I can give this small extention: https://addons.mozilla.org/en-US/firefo ... -playback/What should I change in .xpi to make it compatible with Basilisk?

Basilisk only supports web extensions in gecko-targe mode. Since this extension does not specifically targets Gecko it appears to be corrupt from the browser point of view. You would probably need to add the "applications": { "gecko": { /* ... */} } section with an id and maybe strict_min_version attributes in the manifest. See here for more information about those attributes.

and removed strict_min_version. As a result, the extension is successfully installed, but clicking on its button results in an empty popup page. I guess that the error is in popup/popup.html but can't find out where exactly it is. Could you help me?